1. Field of the Invention
The present invention relates to an illumination device that uses a surface-emitting light source typified by an organic EL (electro luminescence) element.
2. Description of the Related Art
The history of illumination devices is long, and many types have been used conventionally, such as filament light bulbs as point light sources, and fluorescent lights as line light sources. Furthermore, such an illumination device has been proposed, that has a structure in which a multitude of LEDs are arranged in a plane to simulate a surface-emitting light source. In illumination devices using the aforementioned types of light sources, each light source must be mechanically held and accessory components such as a socket must be equipped to supply power to each light source individually, which leads to a problem that the cost and the size of the illumination device are increased.
Recently, organic EL elements that function as surface-emitting light sources have been attracting attention. Such organic EL elements are driven with a low voltage, resulting in a high luminance efficiency, and have reduced weight and thickness, so that the organic EL elements are used in flat panel displays (FDP) in some of portable devices. Furthermore, devices using the organic EL elements as surface-emitting light sources for backlights in liquid crystal display elements, for example, are provided.
On the other hand, organic EL elements can emit lights of R (red), B (blue), and G (green) according to a selection of materials used in the EL light emitting layer. Accordingly, by using each of the emission colors singly or a combination of two or more of the emission colors, a white color or an emission color similar to white can be achieved. Therefore, the organic EL element configured as a surface-emitting light source (light emitting panel) can be used for decorative light sources, high-efficiency light sources for illuminating a room, or the like.
Use of organic EL elements as a surface-emitting type illumination device taking advantage of the aforementioned beneficial properties of organic EL elements is disclosed in Japanese Patent Application Laid-Open No. 2003-7450 and Japanese Patent Application Laid-Open No. 2004-234868.
In Japanese Patent Application Laid-Open No. 2003-7450 and Japanese Patent Application Laid-Open No. 2004-234868, however, the main objects are to prevent generation of uneven brightness in a case where the organic EL element is used as the surface-emitting light source, and merely the use of the organic EL elements as a simple surface-emitting light source is disclosed.